Method for determining the paper quality for halftone printing

ABSTRACT

The invention relates to a method for determining the paper quality for halftone printing, according to which a finely distributed pattern of geometric figures is applied to the paper, said paper is then illuminated and the light that is reflected and scattered by the paper is observed. The method is characterized in that the pattern is applied to an optical transparent planar element, the side of said element bearing the pattern is brought into contact with the surface of the paper and the paper is pressed against the planar element.

BACKGROUND OF THE INVENTION

The invention relates to a method of determining the paper quality forautotypical halftone printing, in which a finely distributed pattern ofgeometric figures is applied to the paper, the paper is illuminated andthe light reflected and scattered by the paper is observed.

During printing, the problem occurs that gray tones can be reproducedonly poorly. In order nevertheless to be able to produce prints of goodquality with graduated lightness tones, use is therefore made to awidespread extent of the technique of autotypical halftone printing, asit is known. In this case, the image is no longer determined bycontinuous lightness transition. Instead, the image is built up from afinely distributed pattern of geometric figures, which are normallydots. In a widespread printing technique, these dots are arrangedregularly in a grid. Depending on the lightness value at a specificpoint of the image to be printed, these dots are made larger or smaller.If the points are arranged sufficiently densely and if the printed imageis viewed from a sufficient distance, then the individual dots are nolonger perceived, but the impression is given of an image with differentgray tones or, if dots of different colors are used, of various hues. Incertain applications, the dots can also be arranged irregularly, thespacing or the number of dots determining which gray tones or hues areperceived by the observer.

The size of such a dot can be calculated in accordance with thelightness of the image point. At the same time, it is necessary to takeaccount of the fact that the dot will in practice assume a greater areaor the printed image perceived by the eye will be more influenced thanwould have to be the case in theory. One problem in printing technology,which depends on the paper quality of the printing technology and otherfactors, consists in the fact that, during printing, the dot becomesphysically larger than is intended. In particular in the case of printsof high quality, it is of course necessary to take this into account.For such prints, use is normally made of coated paper in which, by meansof appropriate surface configuration, it is ensured that the dot doesnot become significantly larger during printing than is intended. Here,another problem of the apparent enlargement of the dot occurs. Thecoating is not completely opaque. Instead, the light penetrates into thecoating down to a certain depth and into the color layer lyingunderneath and is then scattered back. The light scattered back does notalways reach the eye of the observer, however. If a light beampenetrates under an oblique angle into the coating very close to theedge of a dot, it can be scattered under the dot and no longer emergefrom the paper. Light beams which enter close to the dot therefore tosome extent do not emerge from the coating again, so that the dotappears larger than it actually is; the edge becomes “blurred”, andexhibits a shadow (which is referred to as halation).

There are mathematical models relating to how this dot gain can bedetermined (Dissertation “Dot Gain in Colour Halftones” by StefanGustavson, Linköping, September 1997). This actual or apparentenlargement of the dots can be taken into account on the basis of thismathematical model or else on the basis of optical observation of thefinished print, so that theoretically satisfactory prints can beproduced. However, this is a purely theoretical conclusion. This cannotbe implemented technically, since no printed dots matched appropriatelyto the respective location can be produced.

However, it has been shown that the optical surface impression of acoated paper is not completely uniform. For example, the paper can havea nonuniform gloss which, of course, leads to a nonuniform intensity ofthe reflected or scattered light when the print is viewed. The lightnessor white color of the coating can be different, which can be caused by acertain blackening, visible fibers of the paper which are notsufficiently covered by the coating and/or a nonuniform distribution ofoptical whitening agents. The hiding power or opacity of the coating canalso be nonuniform, which can be related with the preceding causes, sothat light beams penetrate into the paper structure to different depths.In addition, the thickness of the coating can vary locally, which leadsto different scatter in the coating and also to nonuniform enlargementof the perceived image of the dot.

All these imperfections of the paper lead to different dot gains. Itwill be possible to detect these to a more or less greater extent whenthe print is observed, when the latter has been finished. This is wherethe present invention starts, it being unsatisfactory, of course, forthe deficient quality of the paper to be detected only after thecomplicated production of the print. In particular, of course a papermanufacturer cannot apply such prints intermediately for test purposesin the reels of paper supplied by him.

The object of the invention consists in providing a method of the typementioned at the beginning with which the quality of the paper can bedetermined simply and reliably before printing and without such a print.

SUMMARY OF THE INVENTION

The solution according to the invention is that the pattern is providedon an optically transparent surface element, the latter is brought flatinto contact with the paper with the side on which the pattern islocated, and the paper is pressed against the surface element.

A determination of the paper quality is therefore carried out without aprint having to be made. Although, in theory, the aforementioneddissertation by Gustavson has proposed imaging a screen onto the paperand observing the light scattered back through the screen, in orderthereby to determine the apparent enlargement of the dots, the screendoes not touch the paper in this case. Here, as the author himselfadmits, these are purely theoretical considerations which have not beenchecked experimentally. It is necessary to ask here how the effect of alight beam diffusing under a colored dot and therefore no longer to beobserved is to be simulated if no single light beam which emerges fromthe coating again is actually absorbed at the surface of this coating.In addition, this theoretically proposed method is not used to determinethe quality of paper, in particular quality fluctuations, but forchecking mathematical models of the aforementioned apparent enlargementof the dots. The dissertation also mentions a method in which a film, onwhich the appropriate dots are produced, is laid on the paper. However,according to the statements of the author of the aforementioneddissertation, this method, with which the dot gain is again to beexamined, suffers from the fact that surface reflections occur which arenot present in a normal print, which falsifies the result. In addition,this method was obviously used only for the theoretical examination ofthe dot gain mechanism, not for determining the paper quality.

A similar method is described in the intermediate report relating to theAiF research project No. 12395N from the Paper Making Institute at theTechnical University of Darmstadt. There too, however, the intention ismerely to examine light scattering and absorption of the paper on theimage reproduction of printed halftone areas, that is to say to examineand check the above effects. The fact that the paper quality, inparticular also nonuniformities in the paper quality which would have ahighly detrimental effect on the print can therefore also be examinedcannot be gathered from the citation. The examination by the Universityof Darmstadt relates to the general effect of the light trappingphenomenon, while our invention measures the variation and makes itvisible. The research project is used for the first practical researchinto the mathematical models relating to general optical properties ofvarious raw materials.

According to the invention, it has now been found that, even without theproof prints previously necessary, the paper quality can be determinedin a quite simple way before printing. The problems with the film laidon in the prior art because of reflections at additional surfaces do notoccur or have no influence, since in the case of completely uniformphotographic film or the like, the reflection conditions are the sameeverywhere. Since no absolute values for the dot gains are to bedetermined, the method operates satisfactorily, simply and accurately inspite of the prejudices mentioned in the dissertation. It is suspectedthat the problems of the prior art do not occur either in the methodaccording to the invention because the paper is pressed very uniformlyagainst the optically transparent surface element.

The pattern on this optically transparent surface element is uniform. Ifthe paper quality is different over the region of this surface element,differences in the lightness or, in the case of colored dots, possiblyalso in the color will be detected by eye immediately. In this way,faults in the paper will be detected significantly better than if animage is printed on it which intrinsically already has differentlightness graduations and is influenced by the dot transfer of theprinting process.

The invention therefore provides, in a surprisingly simple way, a veryaccurate and sensitive method of checking the paper quality beforeprinting. The method can be applied not only in the case of the coatedpaper but also in the case of machine-finished paper.

The paper can be pressed against the transparent surface element by aresilient pressing element. In another advantageous embodiment, thepaper is pressed against the transparent surface element by means of adiaphragm to which a pressure difference is applied. Here, the pressuredifference can be effected by a compressed gas source and/or vacuum.

The transparent surface element can be substantially flat. Here,individual sheets can then be laid on the surface element and examined,or the apparatus can be brought into contact with different points on apaper web, in order to determine the paper quality there. In aparticularly advantageous embodiment, however, the transparent surfaceelement is a transparent rotating roll, in which one or more lightsources and light-sensitive elements are arranged, and the paper ispressed against the transparent roll by a further roll with a resilientsurface. In this case, the scattered light is observed by CCD elements,which can of course also be used in the substantially flat transparentsurface elements. The arrangement with the transparent rotating rollmakes it possible, firstly, to monitor a moving paper web continuously,for example during production. To this end, it is not necessary for thewhole of the paper web to examined but, for example, only a strip about5 cm wide, since in this way the cylinder does not give rise to anyparticularly high costs. Since the paper quality is in particular afunction of the stock composition, information about the entire width ofthe paper can be obtained indirectly by means of the measurement in arelatively small strip. In this case, the diameter of the transparentrotating roll is expediently less than the width, since otherwise theroll could run unstably at high speeds. However, the transparentrotating roll can also be used to examine individual sheets in a mannersimilar to that in a fax machine with original feed.

It is important that the dots are pressed firmly against the paper andthere is no spacing between dots and paper, since otherwise radiationwould pass under the dots.

It has proven to be particularly expedient for the geometric figures,that is to say in general the dots, to cover approximately 15–70% of theentire viewing area, in particular approximately 20–50% of this viewingarea. In this way, particularly sharply formed contrasts are obtainedeven in the case of small differences in the paper or coating quality.The geometric figures are expediently black, since then lightnessdifferences can be seen particularly clearly. If, instead, geometricfigures, in particular dots of different colors, are used, then theimage will also exhibit color shadows in the event of fluctuating paperor coating quality.

As already mentioned, in the case of autotypical halftone printing it ispossible to arrange the points irregularly, the spacing of these dotsdetermining the lightness value, possibly together with the dot size.For the method according to the invention, however, it is particularlyexpedient if the dots are screened regularly. It has proven to beparticularly expedient if approximately 40 to 100 geometric figures,that is to say points or lines of dots, are provided per centimeter.

In an advantageous embodiment, the transparent surface element is a filmor a photographic film, which can be provided particularly simply withthe grid of geometric figures or dots. This film or the photographicfilm is then supported by a transparent glass or plastic plate (or thetransparent roll) if the paper is pressed against the film by the vacuumand the diaphragm (or by the resilient roll).

The observation of the different likeness graduations in the event ofchanging coating or paper quality can be carried out by eye. However, ithas proven to be particularly expedient if the observation is carriedout by a CCD camera and the image from the same is analyzedelectronically.

It is not only possible for the paper from continuous production to bemonitored by the method according to the invention. Instead, it is alsopossible to determine how the quality of the paper improves or becomesworse when production parameters are changed. It is further possible tocheck how the quality compares with the quality from earlier productionruns or with a standard. Testing can be carried out both in thelaboratory and in production.

The dots can also be applied without a transparent carrier, for exampleby Letraset.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described by way of example in the following textusing an advantageous embodiment and with reference to the appendeddrawing, in which:

FIG. 1 shows the scattering of light in coated paper which is printed;

FIG. 2 shows the dot gain with a different thickness of the coating;

FIG. 3 shows the basic structure of the measuring arrangement in detail;

FIG. 4 shows the entire measuring arrangement schematically;

FIG. 5 shows a cross section through the parts essential to theinvention and belonging to another apparatus with which the method ofthe invention can be carried out; and

FIG. 6 shows a cross section through parts of the apparatus from FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As FIG. 1 shows, a partly translucent coating 2 which, for example, haspigments 3 is applied to a paper substrate 1 composed of fibers. Lightbeams A, B and C are scattered differently. In this case, the upper partof FIG. 1 shows a black opaquely printed dot 4, while in the lower partthis printed dot 4 is partly transparent to light, like cyan inks, forexample. The light beam A is scattered out of the coating 2 in bothcases and therefore contributes to the lightness of the image at thispoint. In the upper illustration with a black printed dot, the lightbeam B is scattered under this printed dot 4 and therefore cannot leavethe coating 2 again. A somewhat blurred, dark region will thereforeoccur around the printed dot 4, that is to say the dot 4 will appear tobe larger than it actually is. It is thereby not only light beams Cwhich fall directly onto the printed dot 4 which are extinguished butalso light beams which enter the coating 2 close to the dot 4.

In the lower embodiment of FIG. 1, the light beams which enter theprinted dot 4 are not completely extinguished but attenuated. This leadsto the light beam B emerging from the dot 4 only in attenuated form, sothat the edge of the dot 4 appears darker. On the other hand, the lightbeam C will emerge from the coating 2, even if attenuated, which alsocontributes to this dot 4 appearing blurred.

Given different paper qualities or coating qualities, the dot gain willthen be different, as illustrated in FIG. 2. In the case of theleft-hand dot 4, the light beam D is still scattered out of the coating2, so that at this point the coating 2 still appears bright when viewed.In the case of the right-hand dot 4, however, the coating 2 is thinner,so that the light beam D is absorbed. The right-hand dot 4 thereforeappears to be larger, as illustrated in the lower part of FIG. 2 by thedepictions of the dots or the course I of the intensity of the lightabsorption.

According to the invention, different paper and coating qualities canthen be determined by an apparatus which is to be described briefly inconjunction with FIGS. 3 and 4. A film 5, which is provided with dots 6,is applied to the paper 1, 2 (in FIGS. 3 and 4, no distinction is drawnbetween paper fibers 1 and coating 2). As FIG. 4 shows, this film 5 issupported by a glass plate 7. The paper 1, 2 is laid on that side of thefilm 5 on which the dots 6 are located, and is pressed against the film5 by a diaphragm 8, by a vacuum being produced in the space 9 betweenglass plate 7 and diaphragm 8 by a vacuum pump, also indicated at 21,and said vacuum being maintained. Alternatively, a positive pressure(not shown) could also act externally on the diaphragm 8, or the paper1, 2 could be pressed against the film by a resilient plunger (notshown). Observation can be carried out by eye, as shown on the left inFIG. 4. Another alternative consists in performing the observation witha CCD camera 10, which then allows electronic evaluation to be performedin a unit 11.

FIGS. 4 and 5 show an embodiment in which the above-mentioned electronicevaluation is likewise possible. In this case, the surface element isnot flat but a hollow roll 12 of transparent material, in particularglass or plastic. As FIG. 6 shows, this roll is mounted by bearings 13on hollow tubes 14 and can rotate freely. The paper 1, 2 is pressedagainst the roll 12 by a further roll 16, in particular one made ofrubber. The rolls 12 and 16 are either set rotating in the direction ofthe arrows by the movement of the paper web 1, 2; alternatively the roll16 can also be driven by a motor, in order for example to drawindividual sheets for the examinations into the corresponding apparatusand to draw them between the rolls. Light sources 17 and a row 18 of CCDelements are arranged in the hollow roll 12. The outer surface of theroll 12 is provided with the halftone dots indicated at 6. The lightproduced by the light sources 17 and illustrated dashed in the fig.,which is reflected from the paper web 1, 2 is projected onto the row 18of CCD elements by projection optics indicated at 19. By means ofevaluating the signals from the CCD elements, the paper quality can thenbe determined, as in the previous embodiment. As FIG. 6 shows, althoughthe roll 12 rotates, the row 18 of CCD elements, the optical projectiondevice 19 and the light sources 17 not shown in FIG. 6 do not rotate atthe same time but are fixed to the stationary tubes 14, through whichthe signals from the row 18 of CCD elements are also led away via theline 20. The supply of power to the light sources 17 is also provided ina similar way.

1. A method of determining the paper quality for autotypical halftoneprinting, in which a finely distributed pattern of geometric figures isapplied to the paper, the paper is illuminated and the light reflectedand scattered by the paper is observed, characterized in that thepattern is applied to an optically transparent surface element, which isbrought flat into contact with the paper on the side on which thepattern is located, and the paper is pressed against the surfaceelement.
 2. The method as claimed in claim 1, characterized in that thegeometric figures are dots.
 3. The method as claimed in claim 1,characterized in that the paper is pressed against the transparentsurface element by a resilient pressing element.
 4. The method asclaimed in claim 1, characterized in that the paper is pressed againstthe transparent surface element by a diaphragm to which a pressuredifference is applied.
 5. The method as claimed in claim 4,characterized in that the pressure difference is effected by acompressed gas source.
 6. The method as claimed in claim 4,characterized in that the pressure difference is effected by a vacuum.7. The method as claimed in claim 1, characterized in that thetransparent surface element is a transparent rotating roll in which oneor more light sources and light-sensitive elements are arranged, and inthat the paper is pressed against the transparent roll by a further rollwith a resilient surface.
 8. The method as claimed in claim 1,characterized in that the observation of the scattered light is carriedout by CCD elements.
 9. The method as claimed in claim 1 and wherein thepaper has a viewing area, characterized in that the geometric figurescover approximately 15–70% of the total viewing area.
 10. The method asclaimed in claim 1, characterized in that the geometric figures areblack.
 11. The method as claimed in claim 1, characterized in that thegeometric figures are arranged in a regular grid and approximately 40 to100 geometric figures or lines of figures are provided per centimeter.12. The method as claimed in claim 1, characterized in that thetransparent surface element has a film which is supported by one of atransparent glass or a plastic plate or a roll.
 13. The method asclaimed in claim 1, characterized in that the observation is carried outby the human eye.
 14. The method as claimed in claim 1, characterized inthat the observation is carried out by a CCD camera and the image fromthe CCD camera is analyzed.
 15. The method as claimed in claim 2,characterized in that the dots are applied without a separatetransparent carrier.
 16. The method as claimed in claim 9, characterizedin that the geometric figures cover approximately 20–50% of the totalviewing area.
 17. The method as claimed in claim 2, characterized inthat the paper is pressed against the transparent surface element by aresilient pressing element.
 18. The method as claimed in claim 2,characterized in that the paper is pressed against the transparentsurface element by a diaphragm to which a pressure difference isapplied.
 19. The method as claimed in claim 2, characterized in that thetransparent surface element is a transparent rotating roll in which oneor more light sources and light-sensitive elements are arranged, and inthat the paper is pressed against the transparent roll by a further rollwith a resilient surface.
 20. The method as claimed in claim 12,characterized in that the film is a photographic film.